Many types of machinery, particularly but not only rotating machinery, include components which incur high levels of thermal stress and wear that limit the useful life of the component. This necessitates periodic shut down of equipment for inspection, maintenance and repair, often requiring that components be refurbished or replaced. Turbine blades are exemplary. In many instances the blades are multi-layer structures comprising a casting (a base metal alloy) over which various coatings are formed to extend the useful life by modifying the surface properties of the blade. In some instances, as in blade designs for the forward-most stages of steam and gas turbines, because the blades run at higher temperatures than blades positioned in later stages, it is desirable to coat the blades with a high temperature top coating. This top-most coating, often a metal ceramic material, is affixed to the base metal through an intermediate bond coat. Generally such coatings are applied with techniques such as High Velocity Oxi-Fuel (HVOF) spray coating processes or low pressure plasma sprays. The blades positioned in lower temperature stages may also include a coating for enhancing durability. The coating thickness influences life expectancy of the component. Quality and thickness of an intermediate bond coat can affect the integrity of an overlying thermal barrier.
During the manufacture of turbine blades it is very important to assure uniformity and consistency of coating thicknesses throughout the workpiece and among blades having the same design specification. Otherwise a blade may be subject to premature wear-out and spallation. Coating deposition processes must therefore be designed and characterized to assure that tolerance limits are met throughout each coating and that repeatability exists for the provision of uniformity within acceptable limits of variation from blade to blade. It is therefore also important to monitor processes in order to identify and limit process drift. The foregoing has required costly and time intensive monitoring which has only been effected on a limited basis with mechanical and acoustical techniques. Some techniques require destructive testing such as by forming cut-outs through component layers to measure coating thickness in cross section. Ultrasonic probe techniques have also been applied to measure thicknesses and determine variations but these have been based on a limited number of sample measurements along a coating surface. These measurements have been time intensive. Further, acoustical characterizations may at times be subject to relatively large, undesirable measurement errors when the coating is relatively this, e.g., less than 500 microns thick. Moreover, there remains question as to what kind of variation exists between probed points from which data samples are acquired.
Generally it is desirable to develop techniques which enable an increased level of data sampling to characterize variations in coating thicknesses, to characterize coating processes generally, and to monitor the processes so that trends are identified before deviations extend beyond process windows.